Amputation is the surgical removal of part or all of a limb to address trauma, tumor, disease, and congenital defects. The surgeon must find a balance between two main goals: removal of the diseased or damaged portion of the limb, and reconstruction of the remaining, or residual, limb to promote healing and maintain functionality of the limb. Amputation of the lower limb is the most prevalent major amputation surgery. As of 2005, there were 623,000 Americans living with lower limb amputations, either above or below knee. Most lower limb amputations are a consequence of disease; however, trauma and cancer are significant etiologies and account for 19% of the population.
Transfemoral amputees have amputations above their knee, which means that the knee is amputated as well as parts of the leg below the knee. A basic principle of amputation is conservation of the residual limb. When a transfemoral patient is a candidate for a prosthesis, the level of amputation is determined based on a number of factors, including prosthesis attachment and fitting, and maintaining the functional ability of the patient. As a general rule, the higher the level of transfemoral amputation, the less satisfactory the functional outcome with a prosthesis because the patient loses musculature to control the femur. Furthermore, the length of the residual limb has a significant effect on gait. Transfemoral amputees walk at reduced speeds at least 30% slower than those without amputation.
For lower limb amputees, conventional prostheses attach using a socket that suspends the prosthesis from the residual limb using negative pressure, or suction. Though this is a standard care option, many drawbacks exist, including skin-related issues (e.g., blisters, cysts, skin irritation), and compromised control of the prosthesis. The volume of a limb fluctuates throughout the day and can necessitate the removal or addition of a cloth layer, or sock, over the limb to compensate. Moreover, weight changes throughout the year can necessitate a socket revision.
Direct skeletal attachment, which can involve anchoring to the bone, is an alternative method for prosthetic attachment. Direct Skeletal Attachment (DSA) is the connection of a prosthesis directly to the bone of a residual limb. Attaching directly to the bone has both benefits and risks associated with the procedure. Attaching to the skeletal system eliminates the use of a socket. In doing so, DSA avoids skin related socket complications, allows easier donning and doffing of the prosthesis, provides greater comfort while seated, and limb volumetric change is no longer an issue. Additionally, patients have improved proprioception and sensory feedback through their residual limb, referred to as “osseoperception”. Moreover, DSA can eliminate expenses associated with sockets. Despite numerous advantages of the procedure, challenges still remain. These challenges include infection, bone loss, bone fractures, loosening of hardware, and breakage of percutaneous parts. In some cases, revision surgeries may be required. The stiffness of a titanium implant is much higher than that of the surrounding compact bone, so there is a potential for bone fracture near the osteotomy due to a high external load (e.g., a fall scenario). The area above the implant is most susceptible to fracture, and there is also a risk of longitudinal bone fracture along the length of the implant. One finite element study found that bone failure was three times more likely to occur with a DSA implant than in an intact femur. In a separate numerical simulation study of DSA implants, the safety factor against bone failure was relatively low, even for level-ground walking
Osseointegration, a type of DSA, refers to a direct structural and functional connection between ordered, living bone and the surface of a load-carrying implant. An implant is regarded as osseointegrated when there is essentially no relative movement at the site where the implant contacts the bone. This phenomenon is possible with a titanium implant because the bone tightly adheres to the titanium oxide surface layer of the implant instead of a layer of connective tissue forming between the bone and implant. This structural organization implies that the implant does not induce any inflammatory response. Amputees who benefit most from osseointegration can be those who are unable to tolerate a socket prosthesis, have short residual limbs, or are bilateral transfemoral amputees.
The OPRA protocol (Osseointegrated Prostheses for the Rehabilitation of Amputees) is a transfemoral osseointegration implant system. The OPRA protocol involves a two-stage surgical operation and consists of three components: the fixture, the abutment, and the abutment screw. In the first stage, the medullary cavity of the femur is hand drilled (to minimize bone heating) and a threaded self-tapping titanium fixture is screwed into the cavity. To provide ample time for osseointegration, the second surgery is performed six months later and consists of attaching a percutaneous component, the abutment, to the fixture using the abutment screw. The second procedure is followed by 6-12 months of rehabilitation during which the abutment is progressively loaded and the patient learns to recognize the pain when the implant system is overloaded.